Gas turbines are employed in many fields for driving generators or machinery. In this process, the energy content of a fuel is used to generate a rotational motion of a turbine shaft. For this purpose, the fuel is burnt in a combustion chamber, with compressed air being supplied from a air compressor. The working medium at high pressure and at high temperature generated by the combustion of the fuel in the combustion chamber is conducted, in this process, via a turbine unit connected downstream of the combustion chamber, where the gas expands with an output of work.
In order to generate the rotational motion of the turbine shaft in this process, a number of rotor blades, which are usually combined into blade groups or blade rows, are arranged on this turbine shaft and these rotor blades drive the turbine shaft via a transfer of inertia from the flow medium. In order to conduct the flow medium within the turbine unit, furthermore, guide vane rows connected to the turbine casing are usually arranged between adjacent rotor blade rows. The turbine blades/vanes, in particular the guide vanes, usually have a blade/vane aerofoil extending along a blade/vane axis to appropriately conduct the working medium. A platform extending transverse to the blade/vane axis can be formed at the end of the blade/vane aerofoil for fastening the turbine blade/vane to the respective support body.
In the design of such gas turbines, a usual design objective—in addition to the achievable power—is a particularly high efficiency. For thermodynamic reason, an increase in the efficiency can fundamentally be obtained by increasing the outlet temperature with which the working medium flows out of the combustion chamber and into the turbine unit. In consequence, temperatures of approximately 1200° C. to 1300° C. are an objective for such gas turbines and are also achieved.
In the case of such high temperatures of the working medium, however, the components and structural parts exposed to this working medium are subjected to high thermal stresses. In order, nevertheless, to ensure a comparatively long life of the components affected at a high level of reliability, cooling is usually provided for the components affected, in particular for the rotor blades and/or guide vanes of the turbine unit. The turbine blades/vanes are therefore usually designed so that they can be cooled, it being particularly necessary to ensure an effective and reliable cooling of the first blade/vane, viewed in the flow direction of the working medium. For cooling purposes, the respective blades/vanes in this arrangement usually have a cooling medium duct, which is integrated into the blade/vane aerofoil or the blade/vane profile and from which a cooling medium can be specifically conducted to the thermally stressed zones, in particular, of the turbine blade/vane.
In this arrangement, cooling air is usually employed as the cooling medium. This cooling air is usually supplied to the respective turbine blade/vane, in the manner of an open cooling system, via an integrated cooling medium duct. After emerging from the turbine blade/vane, the cooling air is then mixed with the working medium conducted within the turbine unit. The design power of a gas turbine cooled in this manner is, however, limited, particularly because—in view of the limited mechanical load-carrying capability of individual components of the gas turbine—a further increase in power is usually only achievable by an increased supply of fuel. This, in turn, involves a relative increase in the cooling medium requirement for cooling the turbine blades/vanes, which in turn signifies losses in the available compressor mass flow. These losses can, in turn, only be accepted to a limited degree. In gas turbines, furthermore, it can be necessary, in view also of a necessary level of security, to prevent mixing of cooling medium flowing out of the turbine blade/vane with a working medium flowing through the turbine unit.